1. Field of the Invention
The present invention relates to an optical disc medium for use in carrying out recording/reproducing of information using an optical minute spot and an optical disc apparatus for recording and reproducing thereof.
2. Related Background Art
In an optical recording, recording or reproducing of information is carried out by forming a minute spot using an object lens on a disc recording surface. A recording/reproducing characteristic is determined by a size of the minute spot and by a length of a recording pit formed on the recording surface.
It is known in the art that the size of the spot is in proportion to a wavelength λ of an optical source and is in inverse proportion to a numerical aperture NA of an object lens. When recording pits formed in the optical disc have a short cycle, an amplitude of a reproduced signal obtained as variations of a reflected light beam from the object lens becomes gradually small and becomes zero at 0.5×λNA because the reproduced signal is cut off.
FIG. 1 is a graph showing a relationship between a pit cycle in the optical disc and the amplitude of the reproduced signal. In FIG. 1, the abscissa represents the pit cycle and the ordinate represents the amplitude of the reproduced signal.
In almost all recording methods of the optical disc, front edges or rear edges of pits correspond to transitions between “1” and “0” of a train of encoded data. Accordingly, in almost all recording methods of the optical disc, a shortest pit length is equal to a shortest length between the pits. As a result, a half of the pit period shown in FIG. 1 is equal to a pit length. In this event, the pit length of a cut-off is equal to 0.25×λ/NA.
Conventionally, compact discs (CDs) and digital versatile discs (DVDs) are widely used as the optical discs. In such conventionally widely used optical discs, reproduction of data is carried out by binary equalizing a reproduced signal and by determining presence or absence of the pits using a suitable slice. Accordingly, if an amplitude of the reproduced signal at a shortest pit is not at least level, it is impossible to sufficiently ensure reliability of reproduced data.
FIG. 2 shows measured examples of the shortest pit length and an error rate of the reproduced signal. In FIG. 2, the abscissa represents the shortest pit length(×λ/NA) and the ordinate represents the error rate of the reproduced signal. A broken line in the FIG. 2 shows a conventional example. The conventional example is a case where data reproduction is carried out by binary equalization. In the conventional example from FIG. 2, it is understood that the error rate becomes drastically worse when the pit length is less than 0.35 λ/NA. A dot-dash-line in FIG. 2 shows a standard of the error rate which can be allowed practically. As a result, a limit of a practical shortest pit length in the conventional binary equalization becomes about 0.35 λ/NA before and after the error rate becomes drastically worse in consideration of a margin of a device. For instance, about 0.37 λ/NA is used in the DVD.
In recent years, as a technique for increasing high density recording of an optical disc, a reproduced signal detection method called partial-response maximum-likelihood (PRML) has started to be introduced. PRML is characterized by equalizing the reproduced signal into a multiple-valued signal such as partial-response equalization without equalizing the reproduce signal into a simple binary signal. For example, FIG. 3 shows a waveform example of the reproduced signal in a case of waveform equalizing the reproduced signal into a class of a partial-response called PR (1, 2, 2, 2, 1). In this class, the reproduced signal is equalized into a nine-valued level showing an arrow of a multiple-valued equalized level 5 in this figure. Furthermore, the multiple-valued equalized level is demodulated into a series of data signals which is in most cases by Viterbi decoding in accordance with regularity of temporal transition between the multiple-valued levels. As a result, it is possible to obtain a sufficiently practical reproduction characteristic in an area where the amplitude of the reproduced signal from the shortest pit is small. A solid line of FIG. 2 shows a surveyed example of a reproduce error rate in a case of using PRML. When PRML is used, it is understood that a good characteristic is obtained up to a short pit length compared with the conventional art.
When PRML is used, it is necessary to precisely control an amplitude value of the reproduced signal and a characteristic of a filter used in equalization. This is because the reproduced signal is equalized into a multiple-value without the simple binary value.
In almost all optical disc media, various information related to recorded data are recorded in a particular area. The particular area is called a system information recording area which is set with a general data recording area separated. If an amplitude characteristic of the recorded data and so on are recorded in the system information recording area as this information, it is possible to realize stable recorded data reproduction by setting a circuit according to the information on reproducing by using the optical disc apparatus.
However, in a format of the optical disc medium until now, the information is recorded in the system information recording area under a condition of recording density which hardly changes from recording density of data in the data recording area. Therefore, in order to read information from the system information recording area, a precise setting of operational parameters for a PRML circuit is required from the start. As a result, trial and error of a certain amount of setting values is unavoidable in order to realize stable signal reproduction.